1. Field of the Invention
The present invention relates to a regulator circuit, and in particular, to a regulator circuit having a function of regulating a temperature gradient.
2. Description of the Related Art
FIG. 9A and FIG. 9B are respectively a circuit diagram showing a conventional regulator circuit 900 and a drawing showing the temperature gradient of the output voltage of the conventional regulator circuit. As shown in FIG. 9A, the conventional regulator circuit 900 is structured from a reference voltage output unit 930 which is connected to a power source voltage 910 and outputs a first reference voltage 921, and an amplifying unit 950 to which the first reference voltage 921 is input and which outputs an output voltage 940.
The reference voltage output unit 930 is formed from a current source 931 which supplies a current value I9, a first variable resistor 932 whose resistance value is set to R91, and a rectifying element 933. VDD potential, which is the power source voltage 910 of the regulator circuit 900, is supplied to the positive terminal of the current source 931. The negative terminal of the current source 931 is connected to the positive terminal of the first variable resistor 932 and to the amplifying unit 950. The positive terminal of the first variable resistor 932 is, as described above, connected to the negative terminal of the current source 931 and to the amplifying unit 950. The negative terminal of the first variable resistor 932 is connected to the positive terminal of the rectifying element 933. Further, the positive terminal of the rectifying element 933 is, as described above, connected to the negative terminal of the first variable resistor 932, and VSS potential which is ground potential is supplied to the negative terminal of the rectifying element 933.
The amplifying unit 950 is formed from an operational amplifier 951, a second variable resistor 952 whose resistance value is set to R92, and a resistor 953 having a resistance value R93. The positive terminal of the operational amplifier 951 is, as described above, connected to the negative terminal of the current source 931 and to the positive terminal of the first variable resistor 932. The negative terminal of the operational amplifier 951 is connected to the negative terminal of the second variable resistor 952 and to the positive terminal of the resistor 953. The output terminal of the operational amplifier 951 is connected to the positive terminal of the second variable resistor 952, and outputs the output voltage 940 to an external circuit. As described above, the positive terminal of the second variable resistor 952 is connected to the output terminal of the operational amplifier 951 and to the external circuit, and the negative terminal of the second variable resistor 952 is connected to the negative terminal of the operational amplifier 951 and to the positive terminal of the resistor 953. The positive terminal of the resistor 953 is, as described above, connected to the negative terminal of the operational amplifier 951 and the negative terminal of the second variable resistor 952. The VSS potential which is the ground potential is supplied to the negative terminal of the resistor 953.
Here, by using FIGS. 9A and 9B, the operation of the conventional regulator circuit 900 will be described by using, as an example, a case in which the regulator circuit 900 outputs the output voltage 940 of 1.0 V at 25° C.
In the example of this case, the rectifying element 933 is a diode, and as the temperature characteristic of this diode, the voltage is in the vicinity of 0.6 V at 25° C. and has a temperature gradient of −2.0 mV/° C., as shown in FIG. 9B.
The first reference voltage 921 becomes a value equal to the sum of the output voltage of the rectifying element 933 and the differential voltage between the current source 931 and the first variable resistor 932 {(first reference voltage)=(voltage of rectifying element)+(voltage of first variable resistor)}.
The temperature gradient of the first reference voltage 921 is a value equal to the sum of the temperature gradient of the first variable resistor 932 and to the negative temperature gradient of the rectifying element 933. Generally, a resistor has a positive temperature gradient because, when the temperature increases, the resistance value of the resistor also increases. The higher the ratio of the voltage of the first variable resistor 932 to the voltage of the rectifying element 933, the more the value of the temperature gradient of the first reference voltage 921, which is output from the reference voltage outputting unit 930, is regulated toward the positive direction. Namely, if the resistance of the first variable resistor 932 is made to be large, the value of the temperature gradient of the first reference voltage 921 can be controlled toward the positive direction. In this example, the first variable resistor 932 is set to be 0.3 V at 25° C., and has a temperature gradient of 0.6 mV/° C. Accordingly, the first reference voltage 921 is 0.9 V at 25° C., and has a temperature gradient of −1.4 mV/° C.
The amplifying unit 950 is a non-inverting amplifier circuit structured from the operational amplifier 951, the second variable resistor 952, and the resistor 953. Because the first reference voltage 921 is 0.9 V, by making it be 10/9 times, the output voltage 940 of 1.0 V is obtained. At this time, the output voltage 940 of 1.0 V is obtained by making the ratio of the resistance values of the second variable resistor 952 and the resistor 953 be 1:9. In this case, because the first reference voltage 921 is amplified 10/9 times, the temperature gradient thereof as well is amplified 10/9 times. Namely, the temperature gradient of the output voltage 940 becomes −1.55 mV/° C.
Further, a regulator which controls the temperature characteristic is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 11-121694. Here, the reference voltage generating circuit is structured from a BGR circuit and an output correcting circuit. The BGR circuit has plural resistors and diodes, and outputs a first voltage as the output voltage from the BGR circuit. The output correcting circuit generates a reference voltage by non-inverting-amplifying the first voltage as a first input voltage. By this reference voltage generating circuit which is structured from the BGR circuit and the output correcting circuit, not only the output voltage, but also the temperature dependency can be minimized.
In a regulator circuit such as shown in FIG. 9A, as described above, if the voltage of the first variable resistor 932 is increased, the temperature gradient of the output voltage 940 can be controlled toward the positive direction. However, in order to output the output voltage 940 of 1.0 V at 25° C., the first reference voltage 921 must be made to be less than or equal to 1.0 V at 25° C. For the first reference voltage 921 to be less than or equal to 1.0 V at 25° C., because the voltage of the rectifying element 933 is 0.6 V at 25° C., the voltage of the first variable resistor 932 must be made to be less than or equal to 0.4 V at 25° C.
Further, with regard to the diode which is the rectifying element 933, because there is little dispersion amongst individual diodes, the diode is used as the reference in designing the reference voltage and the temperature gradient, and therefore, diodes cannot be substituted by another element. Accordingly, because the voltage of the first variable resistor 932 is limited from limitations on the output voltage 940, it is difficult to control the temperature gradient of the output voltage 940 toward the positive direction. Further, a regulator which uses a band gap reference circuit, such as that in aforementioned JP-A No. 11-121694, generally requires current of about several μA, and therefore, it is difficult to reduce the current which is consumed.
Moreover, in a case in which a regulator circuit is used in a driver of an LCD or the like making the temperature gradients of the respective circuits which are used in the LCD or the like small is necessary in order for the screen display to not vary greatly in accordance with the temperature. Further, also in cases in which the temperature gradients of the circuits provided at a panel, a display device or the like and the temperature gradient of the regulator circuit are separated, the difference in the contrast of the display luminance of the screen is marked depending on the temperature. In order to overcome these drawbacks, it is preferable for the temperature gradients of the circuits provided at the panel, the display device or the like, and the temperature gradient of the regulator circuit, to match. Therefore, the temperature gradient of the regulator circuit must be able to be controlled freely at least within the range of −1 mV/° C. to 0 mV/° C., and preferably in the range of −1 mV/° C. to 1 mV/° C. Further, in these technical fields, there is the demand for decreased electric power consumption.